The most significant forces displayed in a helicopter rotor hub assembly are detailed below
Flapwise or out-of-plane steady and vibratory shear loads are imparted by the rotor blade in a generally vertical direction and may be equated with vertical lift of the helicopter fuselage. Such loads induce high vertical shear loads within the hub structure.
Flapwise steady and vibratory bending loads are similarly generated by vertical lift loads imparted by the rotor blades and induce compressive and tensile loads within the hub structure generally about an axis in the horizontal axis of the hub. This axis is defined as the flapwise bending neutral axis.
Edgewise or generally in-plane steady centrifugal loads are imparted by the centrifugal forces of each rotor blade. These loads may in certain hub configurations generate a purely axial load, and in others a shear and bending load.
The edgewise shear load associated with centrifugal blade forces is radial in direction (in the horizontal plane) and is generally maximum at the point of blade attachment.
The edgewise bending load due to centrifugal blade forces induce compressive and tensile loads into the hub structure generally about an axis parallel to the vertical. This axis is defined as the edge wise bending neutral axis and is considered to be generally at a right angle or 90 degrees from the flapwise bending neutral axis.
In-plane torsional shear loads imparted to the hub structure by the inertia of a rotor disc (primarily during start-up) and aerodynamic drag are low in comparison to the aforementioned primary loading conditions.
Centrifugal loads migrate from one rotor blade across the hub retention plate to an opposing blade and lift and torque loads migrate from the rotor blades across the hub to the drive shaft. Thus, the major loading conditions experienced by the rotor hub are indeed quite different in origin and orientation. Maximum in-plane and out-of-plane loads are generally located in discrete regions of the hub structure, and are typically very close in proximity due to the desirability of maintaining a small design envelope.
Historically, helicopter main rotor hubs have been made of high strength, lightweight, critical metals or alloys. Metal structures offer generally isotropic strength properties (i.e., the shear, bending and axial strength of the material are constant in all directions) hence, the forming of the hub structure whether by forging, machining or other process does not alter its strength properties. Such properties are desirable from both a manufacturing and structural perspective. The fact that metals can be formed by a variety of well-known techniques offers flexibility in the pursuit of low cost manufacturing methods. In addition, the isotropic and ductile properties of metal make them particularly well-suited for transferring load from the rotor blade to the hub structure. Stress concentrations generated at the interface of bolted or other fastened arrangements in metal structures are generally low in comparison to the stress levels experienced across similar fastening means of composite construction. Thus, metal properties are beneficial in manufacturing and for the transfer of load from one structure to another. Although these metal components have performed adequately, there are a number of drawbacks inherent to these materials. Three important areas where these materials possess less than optimum features are weight, availability and damage tolerance.
Weight reduction has always been a consideration in helicopter construction and with increasing fuel costs it has become a primary objective. At present aluminum and titanium are used extensively because of their light weight and strength, however, there is a constant search for lighter and stronger materials. In addition, since these lightweight metals are primarily available through importation, their supply could be interrupted. Furthermore, these metals may not impart a damage tolerance capability to their components. That is, when a metal component starts to weaken, through fatigue or otherwise, cracks are generated. These cracks continue to grow quickly as there is nothing to stop their propagation and the component part may fail completely.
The industry has taken two approaches in order to overcome the shortcomings of such metal components. One is to build a redundant component so that should one fail, the other will allow for safe landing. The second is to overdesign the particular part such that it would have much greater strength than would normally be required under normal circumstances. Both of these approaches add weight to the aircraft as well as increased cost and reliance on critical metals.
Recently, composite materials have been used as replacement parts for many metal components due to their light weight and relatively low cost. For example, composite materials are now being used in main structural components such as main rotor blades and tail rotor assemblies on helicopters. Fiber reinforced composite matrix materials such as those composed of graphite fibers offer even greater strength than the most comparable metal (e.g. Titanium) and exhibit even lower specific weight properties. The predominant weight advantages of such materials resides in the ability to orient the strength of the fibers in the direction of the load.
In addition, composite articles having fibers oriented in at least two directions (e.g., a structure comprised of a multi-laminate stack of woven fabric) may have discontinuous fibers, which can produce residual interlaminar shear stress. The latter, "free edge" effect, is apparent, for example, when fibers within the composite are off-axis (e.g., 0/90 degrees) rather than unidirectional and is due to the longitudinal and lateral strain properties of the fibers. For example, laterally oriented fibers resist the strain of longitudinal fibers when a load is applied in a longitudinal direction. Hence, residual stress is generated within the resin matrix between the fibers. The general solution in the industry has been to provide continuous fibers where possible for mitigating the concentration of shear stress developed along free edges of the structure.
In addition, typically a compromise between the weight advantage composites offer for the ability to competitively manufacture the composite articles has been made. For example, the fibers oriented in one region of a structure, in an effort to utilize a low-cost composite braiding technique in addition to maintaining continuous fibers, may not be optimum (oriented properly) for accommodating loads in an adjacent region and vise-versa. However, it is not always practical to replace a metal component with a composite material due to particular design considerations and shortcomings in the composite physical properties.
Composite rotor hubs have been designed using a laminated structure of fiber reinforced resin (e.g., note British Pat. No. 2,092,541). Commonly assigned U.S. Pat. No. 4,568,245 describes another composite main rotor hub. Such hubs are of such complicated design as to make them costly to fabricate, and having fewer component parts, produce an increased number of failure points.
Thus, there is still a constant effort being expended in this art to develop damage tolerant, relatively inexpensive and lightweight composite components capable of withstanding the highly loaded complex forces developed in a helicopter rotor hub assembly.